The manufacture of semiconductor devices such as integrated circuits typically involves heat treating silicon wafers in the presence of reactive gases at temperatures of from about 250.degree. C. over 1200.degree. C. The temperatures and gas concentrations to which these wafers are exposed must be carefully controlled, as the ultimate devices often include circuitry elements less than 1 um in size which are sensitive to minute variations in the wafer processing environment.
The semiconductor manufacturing industry has typically used either horizontal or vertical carriers made of silicon carbide or siliconized silicon carbide as kiln furniture for the wafers, and these carriers have been designed to hold up to about 50 wafers. When such conventional carriers are used, the processing steps generally involve fairly slow ramp rates of between about 10.degree. C. and 30.degree. C./minute.
However, because of increasingly strict wafer performance and efficiency requirements, the industry has been considering adopting Rapid Thermal Processing (RTP) wafer processing techniques. According to U.S. Pat. No. 4,978,567 ("Miller"), under RTP conditions, the wafers are treated in an environment whose temperature rises from room temperature to up to about 1400.degree. C. in a period of time on the order of seconds. Typical RTP ramp rates are on the order of 600-6000.degree. C./minute. Under such extreme processing conditions, the thermal shock resistances of the materials in this environment are of critical importance.
Miller discloses RTP wafer carriers made of stand-alone CVD silicon carbide, and carriers made of graphite coated with CVD silicon carbide. However, the cost of stand-alone CVD silicon carbide is often prohibitive, while carriers made of graphite coated with CVD silicon carbide suffer from a significant mismatch of coefficients of thermal expansion ("CTE") which makes the composite susceptible to thermal shock.
Siliconized silicon carbide has been considered as a candidate material for kiln furniture in RTP systems. In particular, U.S. Pat. No. 5,514,439 ("Sibley") has disclosed RTP kiln furniture in which siliconized silicon carbide is the material of choice. However, in one test involving a commercially available siliconized silicon carbide ("Si-SiC") material commonly used as kiln furniture in conventional wafer processing, it was found that this Si-SiC material lost 40% of its flexural strength (from 261 MPa to 158 MPa) when subjected to a thermal quench test in which the temperature of the environment surrounding the material dropped from 500.degree. C. to 0.degree. C. nearly instantaneously.
The finding that the above-mentioned siliconized silicon carbide does not have outstanding thermal shock resistance in RTP environments is not surprising. Torti et al, in "High Performance Ceramics for Heat Engine Applications", ASME 84-GT-92, discusses another siliconized silicon carbide material (NC-430) made by a reaction bonding process which reportedly has high thermal shock resistance. However, Torti et al. also disclose that this NC-430 material has a Tc value of only 275.degree. C., which appears to mean that a significant strength reduction occurs if this material is instantaneously subjected to a temperature differential of only 275.degree. C. Weaver et al. in "High Strength Silicon Carbide For Use In Severe Environments" (1973) reports that a hot pressed SiC material comprising 95-99% SiC has a poor thermal shock resistance.
Therefore, there exists a strong need for a siliconized silicon carbide material which has a thermal shock resistance suitable for its use in kiln furniture designed for RTP applications.
In addition to the more strict thermal shock requirements, another trend in the semiconductor manufacturing industry has been the steady decrease in the level of acceptable metallic contamination in the processed wafers. Accordingly, the industry has concurrently required the kiln furniture to be made of increasingly higher purity materials.
As it is known that the "converted graphite" type of silicon carbide has very low levels of metallic contamination, the art has considered making SiC kiln furniture from converted graphite materials. The process of making such converted graphite materials involves exposing a porous graphite body to SiO gas under carefully controlled conditions which allow a 50% replacement of carbon atoms in the graphite matrix with silicon atoms and the ultimate production of a stoichiometric beta-SiC body. JP Kokai Publication No. 1-264969 (1989) ("Tanso") teaches siliconizing one 30% porous SiC material made from converted graphite to essentially full density, and using that siliconized material as a wafer boat in semiconductor wafer processing operations. Tanso further teaches that its essentially non-porous siliconized product made from its process can have a density of from 2.9 g/cc to 3.2 g/cc. Since silicon and silicon carbide have respective densities of 2.33 g/cc and 3.21 g/cc, respectively, Tanso appears to disclose siliconized SiC products having from 64 vol % to 99 vol % silicon carbide. However, the actual enabling technology disclosed by Tanso appears to be limited to only lower SiC fraction bodies. In particular, Tanso teaches that the reason for its successful conversion of graphite to stoichiometric SiC was its decision to limit the density of the graphite starting body to no more than 1.50 g/cc in order to provide enough porous passages within the graphite body to allow complete infiltration of the SiO gas. Since following this suggestion appears to limit the density of the converted SiC body to only about 2.25 g/cc, Tanso appears not to teach how to make a converted graphite SiC body having a density of over 2.25 g/cc (or 70.09 vol % SiC), and so does not further teach a siliconized SiC body having over 70.09 vol % SiC.
One known commercial producer of converted graphite for use in semiconductor wafer processing offers a porous beta-SiC material made from converted graphite and having a density of 2.55 g/cc, or about 80 vol % SiC. However, the reported room temperature flexural strength of this material (25 ksi, or about 175 MPa) is relatively low. Typically, a room temperature flexural strength of at least about 230 MPa is highly preferred for commercially useful SiC diffusion components. Moreover, although it is known that siliconizing a porous SiC body typically enhances its strength, a brochure from the above-mentioned producer discourages siliconizing this porous converted graphite product having 80 vol % SiC for fear of thermal expansion mismatch consequences. In particular, according to the producer's brochure, the difference in the coefficients of thermal expansion ("CTE") between silicon (CTE=2.5-4.5.times.10.sup.-6 /.degree. C.) and silicon carbide (CTE=4.8.times.10.sup.-6 /.degree. C.) is so great that, on cooldown from siliconization, the SiC contracts much more than the silicon, and this creates stresses of the intergrain bonds in the SiC during both cooldown from siliconization and subsequent thermal cycles. Therefore, it appears this brochure actively discourages the siliconization of porous converted graphite products having over 71 vol % SiC for fear of producing strength-degrading cracks in the composite material. Therefore, there is a further need for a siliconized silicon carbide material having over 71 vol % silicon carbide (preferably over 75 vol % SiC, more preferably at least 80 vol % SiC) which has both the higher purity and adequate strength needed for conventional wafer carrier applications, and preferably the high thermal shock resistance required for RTP applications of the future.